Discharge lamp, mount therefor, and method



Oct. 30, 1956 HE|NE ET AL 2,769,112

DISCHARGE LAMP, MOUNT THEREFOR, AND METHOD Filed June 11, 1955 INVENTORSH- f/El/VE and 6750265 IVE/672?.

' IATTORNE A United States Patent O DISCHARGE LAMP, MOUNT THEREFOR, ANDMETHOD Application June 11, 1956, Serial No. 360,980 2 Claims. (31.sis-21s This invention relates to discharge lamps and, moreparticularly, to the construction of the mounts of discharge lamps andespecially those in which electronemission material is fused oncoiled-coil filaments.

The principal object of our invention generally considered, is toeliminate or lessen light-absorbing so-called end bands which form onthe envelopes of discharge lamps, especially those made by the fusedcathode process, by coating only the lead wires, coil clamps, coil tailsand anode wires, that is, all the interior metal parts except thecathodes, with a suspension of zirconium dioxide or other refractoryinsulating oxide.

Another object of our invention is to apply such refractory insulatingoxide to the metal parts, other than the electron-emissive portions, asa suspension in isopropyl alcohol or other mediums sufliciently volatileto evaporate completely with no residue, by electrophoresis, or byspraying or dipping in such insulating material, to break the electricalpath between said metal parts and the arc.

Other objects and advantages of the invention will become apparent asthe description proceeds.

Referring to the drawing, the sole figure is a perspective view, withparts in section and parts broken away, of a fluorescent discharge lampembodying our invention.

Extensive tests have shown that lamps embodying the invention describedand claimed in Patent No. 2,476,590 to George S. Evans, dated July 19,1949, resulted in greatly increasing lamp life when used in applicationswhere frequent instant starting is required. The patent discloses themaking of the so-called fused cathode in producing an electron-emissivecoating on electrodes for discharge lamps. The emissive material thereindescribed is barium oxide, and the life of a fluorescent lamp with abarium oxide fused cathode, as produced by the rapid heating of acoating of barium carbonate thereon, is considerably longer, as comparedwith a lamp utilizing one not fused, particularly where frequent instantstarting is required, as in a flashing sign.

Some improvement in life has also been obtained by the use of bariumoxide and zirconium dioxide as a fused cathode coating material. In theco-pending application of the present applicants, Serial No. 335,908,filed February 9, 1953, there is disclosed an improvement inelectron-emissive material involving the incorporation of some magnesiumoxide. The use of such material has extended the instant start life ofabout 4700 hours obtained using previous materials by about 46.5% toabout 6882 hours, with only mg. of emission material per coil.

Lamps currently made by the fused cathode hold about 20 mg. of emissionmaterial per coil. Such an amount of emission material raises the lamplife above 10,000 hours. Such fusion process, however, has only beenused on lamps designed for special applications in which the maindisadvantage of the process, that is, early end banding, is tolerated togain the advantages of long life. But for the fact that end bands appearafter a relatively short burning period, the process would probably beused on all instant start lamps with a resultant phenomenal increase inlamp life.

In accordance with our invention, early end banding is eliminated orlessened in lamps made by the fused cathode process, as well as in lampsmade by the ordinary non-fusing process. The method essentially consistsin coating only the internal metal mount parts, except the electronemission material coated or cathode portions, that is, such parts as thelead wires, coil clamps, coil tails, and anode wires, with a suspensionof a refractory metal oxide, preferably zirconium dioxide. However, wedo not wish to be limited to this particular material, because otherrefractory or stable oxides, such as magnesia, may be substituted. Theelectron emissive material on the cathode must not be coated with such asuspension, but only the other metal parts.

The suspension desirably employed is finely divided zirconium dioxide inisopropyl alcohol. It is desirably applied by brushing the suspension onto only the required area as a continuous coating. However, othersuspending means may be used, as it is necessary only that the medium besufficiently volatile to evaporate completely and leave no residue.Other methods of applying the material, such as spraying or dipping, canbe used, provided a continuous coating is obtained.

The discovery that such coating of non-electron-v emission portions ofthe internal metal parts of discharge lamps eliminates or reduces endbanding was made after observing that when a freshly exhausted fusedcathode lamp was initially seasoned, an arc struck on the lead wires andanodes and moved about, sparking vigorously, until finally it settled toa hot spot on one of the coil tails outside of the clamp or on the mainbody of the coil. After a short seasoning period, the hot spot in allcases settled on the main body of the coil. On subsequent lamps it wasnoted before initial seasoning that the anode wires and lead wires haddeveloped a partly blue, partly yellow coating during exhaustprocessing. After seasoning the coating had apparently disappeared.

Lead wires oxidized in a gas flame used in non-fused lamps did not causethe arc to strike other than on the coil. However, on burning, suchlamps developed end bands after a short period of operation. From thiswe decided that during the fusing process the nickel lead wires wereeither oxidized or they collected a coating of oxide from oxidizedtungsten coil, and in addition may have collected a small amount ofbarium oxide which acted as an emitter to attract the arc during initialseasoning. The application of the insulating coating, in accordance withour invention, breaks the electrical path between the arc and the metalparts, other than the cathode, thereby solving the problem.

Referring to the drawing in detail, like parts being designated by likereference characters, there is shown a lamp 10 having a translucentenvelope 12, with a base 14 at each end thereof. A cold-hot cathode orelectronemission material-coated electrode 16 is supported on each endof the envelope on lead-in conductors 18 sealed through the press 20 ofthe reentrant stem or flare tube 21. These conductors 18 make electricalconnection with contact prongs 23 in a conventional manner, therebyproviding means for supplying power to the lamp.

This type of cathode is called a cold-hot cathode, since it is startedcold and operated hot. In a preferred embodiment of our invention it isa filamentary type cathode in which the refractory metal wire is woundinto a minor coil which, in turn, is wound into a major coil to make acoiled-coil common in the art. Such a coiled-coil metal filament ispreferably formed of tungsten, but may be of other suitable refractorymetal such as molybdenum. Fused on the inner coil of this coiledcoilrefractory metal filament and adhering thereto is the-electronremissionmaterial, preferably such: as disclosed and claimedin our applicationSerialNo, 335,908; previously referred to.

The sealed translucent envelope 12 encloses. a small amount of-mercury22; to provide-mercury vapor, and afilling; of inertionizable gas, suchas argon, ora mixtureof suchgas with another such as neon, to facilitatestarting A representativepressure of inert ionizable gas is about-3.2mm. and arepresentative operating mercury vapor pressureis. aboutlvmicrons. If v the lamp is one of the fluorescent type, the interior ofthe envelope is.

coated with luminescent material 28.

Fused cathodes used in instant. start fluorescent lamps generallyconsist of alkaline earth oxides, produced by heating thecarbonates,employed singly, or in combination with one or more of the otheralkaline earth oxides. The alkaline earth of. the cathode is preferablybarium oxide, and the cathode is made generally in accordance with theteachings of the aforementioned Evans patent. The cathode mixture orelectron-emission material. is desirably made in accordance with theteachings of application previously referred to.

In accordance withthe present invention, we eliminate or decrease theundesirable formation of end bands by coating the interior metal partsof the mountconsisting.

of the glass part 21 and t he metal parts extending inwardly therefromincluding the lead-in conductors 18, with a firmly adherent insulatingcoating, preferably from to mils in thickness, comprising a refractoryoxide or oxides, zirconium oxide being a preferred example. End bandsare darkened areas on the inner surface of the envelope, over theluminescent material, if carried thereby, near but between thetransverse planes of the cathodes, which bands gradually-decrease indensity as they approach one another, one of said areas beingrepresented at 24-. This coating may be applied as a suspension inisopropyl alcohol, or by other methods as previously mentioned, tospecifically the lead portions 18, the coil clamps 25, the coil tails26- and the anode wires or horns 27, if present.

Two groups of fused cathode lamps were made up, each using the sameemission material, emission material Weight, and exhaust schedule, withthe essential difference between the two groups being that in. one casethe lead Wires, coil clamps, coil tails and anodes were coated with asuspension of zirconium dioxide in isopropyl alcohol, and in the othercase said parts were not so coated. All of the noncoated lamp endsdeveloped faint end bands after burning for about hours, Which bandscontinued to become darker, while only one end of one of thecoated-mount lamps developed a light band after burning forabout hours,which band did not change thereafter. All other ends of all of thecoated mount lamps were quite clean.

- in one group resulted in an increase in life from 7400 to 8700 hours,that is, 17.5%, and in another group from 5200 to 6350 hours, that is22.1%. The lamps within each group were similarly constructed, exceptfor the protective coating on the metal parts. The Weight of theelectron-emission material was greater in the first group of lamps thanin the second group above noted, but still less than the amount 20 mg.used in lamps in production which average over 10,000 hrs. life.

Although a preferred embodiment has been disclosed, it will beunderstood that modifications may be made within the spirit and scope ofthe invention.

We claim:

1. In a discharge lamp, an envelope, a stem sealed,

to each end thereof, a pair of leads projecting inwardly from said stem,an electrode supported between the leads of each pair and comprising arefractory metal coiled-coil wire with a fused coating ofelectron-emission material thereon, and zirconium dioxide coating offrom 5 to 10 mils thick on the exposed interior metal portions in saidenvelope, to eliminate formation of end bands and to prolong the usefullife of said discharge lamp. H 2. A mount for a discharge lampcomprising a closure member for an end of the envelope thereof, leadsextending inwardly from said closure member, a filamentary electrodecarried by and with tails clamped in said leads, which leads areextended beyond the clamps to form anode wires, electron-emissionmaterial thereon, the exposed portions of said leads, coil clamps, coiltails, and anode wires being coated with zirconium dioxide of from 5 to10 mils thick to prevent the formation of end hands when mounted in adischarge lamp thus prolonging its useful life.

References Citedin the file of this patent UNITED. STATES PATENTS2,146,579 Inman Feb. 7, 1939 2,171,230 Wamsley Aug. 29, 1939 2,233,917De Boer Mar. 4, 1941 2,348,045 Wooten May 2, 1944 2,417,458 Eitel et al.Mar. 18, 1947 2,445,692 Porter et al July 20, 1948 2,491,284 Sears Dec.13, 1949 2,576,129 Levin Nov. 27, 1951 2,682,002 Germer June 22, 1954

